Polarization Ellipse: Understanding 2*Psi Angle

[Steleo]

Hello all

In radio astronomy the orientation angle of an ellipse is usually quoted as
tan(2 *Psi) = S2/S1

Where S2 and S1 are the stokes parameters.

Does anyone know or can point me to a reference as to why the 2 * psi should be in there? The angle of the axis in the ellipse should be able to go between -Pi/2 to Pi/2? Does it have something to do with the fact that you can rotate all your polarisers by 90 degrees and get the same result?

Cheers

 

[BoTemp]

http://en.wikipedia.org/wiki/Stokes_parameters

If you take the polarization vector and rotate it 180 degrees, you are describing the same ellipse. That's where the factor of 2 comes from, to make those functions periodic over 180 degrees instead of 360 degrees. Not sure if that's exactly what you were talking about when you said rotate both of your polarizers 90 deg., but it's the same idea.